This relates generally to integrated circuits, and more particularly, to integrated circuits with wireless communications circuitry.
Integrated circuits with wireless communications capabilities typically include amplifying circuits that are used to amplify the power of radio-frequency signals prior to wireless transmission. For example, a radio-frequency power amplifier may receive input signals having an input power level and generate corresponding output signals having an output power level, where the output power level of the output signal is generally greater than the input power level of the input signal. Ideally, the power amplifier exhibits a perfectly linear input-output power transfer characteristic (i.e., an increase in the input power by a certain amount should result in a corresponding predetermined amount of increase in the output power). In practice, however, power amplifiers often exhibit non-linear behavior. When a power amplifier is non-linear, an increase in the input power may result in a corresponding increase in the output power that is less than the predetermined amount.
Such type of non-linearity issues can degrade signal integrity and adversely impact wireless performance. In an effort to mitigate power amplifier non-linearity, predistortion circuitry has been developed. Predistortion circuitry is formed in the signal path leading up to the input of the power amplifier and serves to “predistort” the signals prior to amplification so as to compensate for any non-linear behavior of the radio-frequency power amplifier. In other words, the predistortion circuitry is used to introduce a signal predistortion effect that cancels out with the non-linear signal transfer characteristic of the power amplifier so that signals are properly amplified at the output of the power amplifier.
In order to perform predistortion, the predistortion circuitry is configured to monitor the input signals received at the input of the power amplifier and the output signals generated at the output of the power amplifier. Arranged in this way, the transfer characteristic of the power amplifier can be analyzed. Predistortion operations generally involve some type of signal alignment operation, coefficient computation operation, and predistortion signal filtering operation. The signal alignment operation serves to align the monitored input and output signals so that comparison between the two signals can be properly performed. The coefficient computation operation analyzes the aligned signals and outputs corresponding coefficients that directly control the predistortion signal filtering operation (e.g., the type of filtering or predistortion being introduced is based on the generated coefficients).
In one conventional arrangement, the predistortion circuitry implements only time domain signal alignment. Time domain alignment (TDA) operations involve a coarse time domain alignment step, an upsampling step, a fine time domain alignment step, followed by a downsampling step. Implementing predistortion in this way is fast but requires a significant amount of memory as sampling rate is increased. Moreover, the fine time domain alignment step is sensitive to sampling frequency and requires re-tuning whenever the sampling frequency changes.
In another conventional arrangement, the predistortion circuitry implements only frequency domain signal alignment. Frequency domain alignment (FDA) operations involve a fast Fourier transform (FFT) operation that converts the monitored input and output signals from the time domain to the frequency domain, a single alignment operation in the frequency domain, followed by an inverse FFT operation that converts the aligned signals from the frequency domain back to the time domain. Implementing predistortion in this way does not require as much memory and is less sensitive to sampling frequency compared to the TDA-based predistortion but is costly as the FFT and iFFT operations require substantially more complex processing circuitry and is substantially slower than pure TDA operations.
It would therefore be desirable to provide improved predistortion circuitry that is reasonably fast, memory efficient, and reduced sensitivity to sampling frequency.